Roger,
I have an electric Sears Weedwacker. The spool is
held onto the shaft by two metal bent tangs. Try
grabbing hold of the spool with one hand and holding
onto the base with the other and begin pulling. MIne comes off with a little muscle. To reinstall, hold at angle and push spool over one tang first and slowly strighten it to align with shaft head. Hope this works for your specific model.
Mike, August 2006